Magnetron microwave tubes are most commonly used as high-power oscillators either in a pulsed or a continuous mode. In a number of applications such as, for example, phased array radar and coherent radar systems, it is necessary to maintain the oscillator output signal locked in phase and frequency to a reference signal. It is well known in the microwave tube art to inject a signal into a magnetron type device to influence its performance as an oscillator. It is also known that the magnetron can be used as a negative resistance in a reflection-type microwave amplifier.
In prior art systems, the input signal is coupled or injected into the output port of the magnetron by means of a microwave circulator. Since the magnetron is not a matched load for the transmission line, a portion of the input signal is reflected from the output port of the magnetron and the remainder is coupled into the tube. The purpose of the circulator is to isolate the injected signal source from reflected input power and from the power output of the magnetron device. The division of input signal between reflected signal power and the useful signal coupled into the tube depends upon the magnitude of the mismatch associated with the resonator circuit coupling.
Magnetron oscillators have been used with signal injection in two ways: to obtain magnetron injection priming or to obtain magnetron injection locking. In magnetron injection priming the input signal is used to influence the starting phase of the oscillator but has little control of the oscillator frequency and phase when it is operating at full power. A relatively low input power level can accomplish control of the magnetron starting phase. Injection priming has had limited application.
In magnetron injection locking, the power level of the input signal is typically much larger than for priming. When the signal coupled into the oscillator tube is sufficiently large and sufficiently close in frequency to the free-running frequency of the magnetron oscillator, both the oscillator frequency and phase have a fixed relationship to the input signal. Useful injection locking has been obtained only at significantly lower gain than for oscillator priming.
It is desirable to control the output of an injection-locked magnetron over a selected bandwidth. However, the fraction of the input power that is coupled into the tube decreases as the input signal departs from the resonant frequency of the oscillator anode circuit, thereby necessitating additional input power. The external Q of the loaded resonant circuit of the oscillator can be decreased in order to increase bandwidth. However, in this case the input power level required to maintain locking also increases. As a result, acceptable bandwidths for practical system applications have been obtained only at moderate gain, and devices such as crossed-field amplifiers have been more widely utilized in system applications.
In order to use magnetron devices as reflection amplifiers, the external Q of the composite, loaded resonant circuit is made much smaller than the internal Q of the oscillator in order to obtain adequate bandwidth. Reflection amplifier operation over bandwidths of one to three percent or more has been achieved. However, the gain and bandwidth combination available with a magnetron device used as a reflection amplifier has not been sufficient to find widespread use. Such devices do not compare favorably with existing crossed-field amplifiers.
In spite of the drawbacks of prior art signal injection techniques, magnetron-type devices offer many advantages over other crossed-field devices including a more noise free output, better phase tracking between input and output, ease of frequency scaling, uniform anode power dissipation, and reduced size, weight and manufacturing costs. Therefore, it has long been an object of research efforts to provide a magnetron-type device wherein the output frequency and phase are locked to an input signal and wherein high gain and wide bandwidth are simultaneously obtained.
It is a general object of the present invention to provide an improved microwave tube.
It is another object of the present invention to provide a microwave tube similar in structure to a magnetron having a directional input port for coupling an input signal into an anode circuit while blocking internally-generated power from passing through the input port.
It is a further object of the present invention to provide a microwave tube similar in structure to a magnetron having a directional input port and a separate output port.
It is still another object of the present invention to provide a microwave tube similar in structure to a magnetron wherein output frequency and phase are locked to the frequency and phase of an input signal at a relatively high level of gain and over a relatively wide bandwidth.
It is still another object of the present invention to provide a microwave tube similar in structure to a magnetron which can be utilized as an amplifier having relative high gain and wide bandwidth.
It is yet another object of the present invention to provide a microwave tube similar in structure to a magnetron having a high output power level.
It is a further object of the present invention to provide a microwave tube having relatively small size and weight and having relatively low manufacturing cost.